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Revised^6 Report on the Algorithmic Language Scheme
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<a name="node_chap_6"></a>
<h1 class=chapter>
<div class=chapterheading><a href="r6rs-Z-H-2.html#node_toc_node_chap_6">Chapter 6</a></div><br>
<a href="r6rs-Z-H-2.html#node_toc_node_chap_6">Entry format</a></h1>
<p></p>
<p>
The chapters that describe bindings in the base library and the standard
libraries are organized
into entries.  Each entry describes one language feature or a group of
related features, where a feature is either a syntactic construct or a
built-in procedure.  An entry begins with one or more header lines of the form</p>
<p>
</p>

<p class=noindent></p>
<div align=left><tt><i>template</i></tt>&nbsp;&nbsp;&nbsp;&nbsp;<i>category</i>&nbsp;</div>
<p>
The <i>category</i> defines the kind of binding described by the entry,
typically either &#8220;syntax&#8221; or &#8220;procedure&#8221;.
An entry may specify various restrictions on subforms or arguments.
For background on this, see section&nbsp;<a href="r6rs-Z-H-8.html#node_sec_5.4">5.4</a>.</p>
<p>
</p>
<a name="node_sec_6.1"></a>
<h2 class=section><a href="r6rs-Z-H-2.html#node_toc_node_sec_6.1">6.1&nbsp;&nbsp;Syntax entries</a></h2>
<p>If <i>category</i> is &#8220;syntax&#8221;, the entry describes a 
special syntactic construct, and the template gives the syntax of the
forms of the construct.  
The template is written in a notation similar to a right-hand
side of the BNF rules in chapter&nbsp;<a href="r6rs-Z-H-7.html#node_chap_4">4</a>, and describes
the set of forms equivalent to the forms matching the
template as syntactic data.  Some &#8220;syntax&#8221; entries carry a
suffix (<tt>expand</tt>), specifying that the syntactic keyword of the
construct is exported with level
1.  Otherwise, the syntactic keyword is exported with level 0; see
section&nbsp;<a href="r6rs-Z-H-10.html#node_sec_7.2">7.2</a>.</p>
<p>
Components of the form described by a template are designated
by syntactic variables, which are written using angle brackets, for
example, &lt;expression&gt;, &lt;variable&gt;.  Case is insignificant
in syntactic variables.  Syntactic variables
stand for other forms, or
sequences of them.  A syntactic variable may refer to a non-terminal
in the grammar for syntactic data (see section&nbsp;<a href="r6rs-Z-H-7.html#node_sec_4.3.1">4.3.1</a>),
in which case only forms matching
that non-terminal are permissible in that position.
For example, &lt;identifier&gt; stands for a form which must be an
identifier.
Also,
&lt;expression&gt; stands for any form which is a
syntactically valid expression.  Other non-terminals that are used in
templates are defined as part of the specification.</p>
<p>
The notation
</p>
<p>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&lt;thing<sub>1</sub>&gt; <tt>...</tt>
</p>
<p>
indicates zero or more occurrences of a &lt;thing&gt;, and
</p>
<p>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&lt;thing<sub>1</sub>&gt; &lt;thing<sub>2</sub>&gt; <tt>...</tt>
</p>
<p>
indicates one or more occurrences of a &lt;thing&gt;.</p>
<p>
It is the programmer&#8217;s responsibility to ensure that each component of
a form has the shape specified by a template.  Descriptions of syntax
may express other restrictions on the components of a form.
Typically, such a restriction is formulated as a phrase of the form
&#8220;&lt;x&gt; must be<a name="node_idx_260"></a>a <tt>...</tt>&#8221;.  Again, these
specify the programmer&#8217;s responsibility.  It is the implementation&#8217;s
responsibility to check that these restrictions are satisfied, as long
as the macro transformers involved in expanding the form terminate.
If the implementation detects that a component does not meet the
restriction, an exception with condition type <tt>&amp;syntax</tt> is raised.</p>
<p>
</p>
<a name="node_sec_6.2"></a>
<h2 class=section><a href="r6rs-Z-H-2.html#node_toc_node_sec_6.2">6.2&nbsp;&nbsp;Procedure entries</a></h2>
<p>If <i>category</i> is &#8220;procedure&#8221;, then the entry describes a procedure, and
the header line gives a template for a call to the procedure.  Parameter
names in the template are <i>italicized</i>.  Thus the header line</p>
<p>
</p>

<p class=noindent></p>
<div align=left><tt>(vector-ref <i>vector</i> <i>k</i>)</tt>&nbsp;&nbsp;&nbsp;&nbsp;procedure&nbsp;</div>
<p>
indicates that the built-in procedure <tt>vector-ref</tt> takes
two arguments, a vector <i>vector</i> and an exact non-negative integer
object <i>k</i> (see below).  The header lines</p>
<p>
</p>

<p class=noindent></p>
<div align=left><tt>(make-vector <i>k</i>)</tt>&nbsp;&nbsp;&nbsp;&nbsp;procedure&nbsp;</div>

<div align=left><tt>(make-vector <i>k</i> <i>fill</i>)</tt>&nbsp;&nbsp;&nbsp;&nbsp;procedure&nbsp;</div>
<p>
indicate that the <tt>make-vector</tt> procedure takes
either one or two arguments.  The parameter names are
case-insensitive: <i>Vector</i> is the same as <i>vector</i>.</p>
<p>
As with syntax templates, an ellipsis <tt>...</tt> at the end of a header
line, as in</p>
<p>
</p>

<p class=noindent></p>
<div align=left><tt>(= <i>z<sub>1</sub></i> <i>z<sub>2</sub></i> <i>z<sub>3</sub></i> <tt>...</tt>)</tt>&nbsp;&nbsp;&nbsp;&nbsp;procedure&nbsp;</div>
<p>
indicates that the procedure takes arbitrarily many arguments of the
same type as specified for the last parameter name.  In this case,
<tt>=</tt> accepts two or more arguments that must all be complex
number objects.</p>
<p>

A procedure that detects an argument that it is not specified to
handle must raise an exception with condition type
<tt>&amp;assertion</tt>.  Also, the argument specifications are exhaustive: if the
number of arguments provided in a procedure call does not match 
any number of arguments accepted by the procedure, an exception with
condition type <tt>&amp;assertion</tt> must be raised.</p>
<p>
For succinctness, the report follows the convention
that if a parameter name is also the name of a type, then the corresponding argument must be of the named type.
For example, the header line for <tt>vector-ref</tt> given above dictates that the
first argument to <tt>vector-ref</tt> must be a vector.  The following naming
conventions imply type restrictions:</p>
<p>
</p>
<table border=0><tr><td valign=top ><i>obj</i></td><td valign=top >any object</td></tr>
<tr><td valign=top ><i>z</i></td><td valign=top >complex number object</td></tr>
<tr><td valign=top ><i>x</i></td><td valign=top >real number object</td></tr>
<tr><td valign=top ><i>y</i></td><td valign=top >real number object</td></tr>
<tr><td valign=top ><i>q</i></td><td valign=top >rational number object</td></tr>
<tr><td valign=top ><i>n</i></td><td valign=top >integer object</td></tr>
<tr><td valign=top ><i>k</i></td><td valign=top >exact non-negative integer object</td></tr>
<tr><td valign=top ><i>bool</i></td><td valign=top >boolean (<tt>#f</tt> or <tt>#t</tt>)</td></tr>
<tr><td valign=top ><i>octet</i></td><td valign=top >exact integer object in {0, <tt>...</tt>, 255}</td></tr>
<tr><td valign=top ><i>byte</i></td><td valign=top >exact integer object in { <tt>-</tt> 128, <tt>...</tt>, 127}</td></tr>
<tr><td valign=top ><i>char</i></td><td valign=top >character (see section&nbsp;<a href="r6rs-Z-H-14.html#node_sec_11.11">11.11</a>)</td></tr>
<tr><td valign=top ><i>pair</i></td><td valign=top >pair (see section&nbsp;<a href="r6rs-Z-H-14.html#node_sec_11.9">11.9</a>)</td></tr>
<tr><td valign=top ><i>vector</i></td><td valign=top >vector (see section&nbsp;<a href="r6rs-Z-H-14.html#node_sec_11.13">11.13</a>)</td></tr>
<tr><td valign=top ><i>string</i></td><td valign=top >string (see section&nbsp;<a href="r6rs-Z-H-14.html#node_sec_11.12">11.12</a>)</td></tr>
<tr><td valign=top ><i>condition</i></td><td valign=top >condition (see library section&nbsp;on &#8220;Conditions&#8221;)</td></tr>
<tr><td valign=top ><i>bytevector</i></td><td valign=top >bytevector (see library chapter&nbsp;on &#8220;Bytevectors&#8221;)</td></tr>
<tr><td valign=top ><i>proc</i></td><td valign=top >procedure (see section&nbsp;<a href="r6rs-Z-H-4.html#node_sec_1.6">1.6</a>)
</td></tr></table>
<p>
Other type restrictions are expressed through parameter-naming
conventions that are described in specific chapters.  For example,
library chapter&nbsp;on &#8220;Arithmetic&#8221; uses a number of special
parameter variables for the various subsets of the numbers.</p>
<p>
With the listed type restrictions, it is the programmer&#8217;s responsibility to
ensure that the corresponding argument is of the specified type.
It is the implementation&#8217;s responsibility to check for
that type.</p>
<p>
A parameter called <i>list</i> means that it is the
programmer&#8217;s responsibility to pass an argument that is a list (see
section&nbsp;<a href="r6rs-Z-H-14.html#node_sec_11.9">11.9</a>).  It is the implementation&#8217;s responsibility
to check that the argument is appropriately structured for the
operation to perform its function, to the extent that this is possible
and reasonable.  The implementation must at least check that the
argument is either an empty list or a pair.</p>
<p>
Descriptions of procedures may express other restrictions on the
arguments of a procedure.  Typically, such a restriction is formulated
as a phrase of the form &#8220;<i>x</i> must be a <tt>...</tt>&#8221; (or otherwise
using the word &#8220;must&#8221;).</p>
<p>
</p>
<a name="node_sec_6.3"></a>
<h2 class=section><a href="r6rs-Z-H-2.html#node_toc_node_sec_6.3">6.3&nbsp;&nbsp;Implementation responsibilities</a></h2>
<p>In addition to the restrictions implied by naming conventions, an
entry may list additional explicit restrictions.
These explicit restrictions usually describe both the
programmer&#8217;s responsibilities, who must ensure that the subforms of a
form are appropriate, or that an appropriate
argument is passed, and the implementation&#8217;s responsibilities, which
must check that subform adheres to the specified restrictions (if
macro expansion terminates), or if the argument is appropriate.  A description
may explicitly list the implementation&#8217;s responsibilities for some
arguments or subforms in a paragraph labeled &#8220;<i>Implementation
responsibilities</i>&#8221;.  In this case, the responsibilities specified
for these subforms or arguments in the rest of the description are only for the
programmer.  A paragraph describing implementation responsibility does not
affect the implementation&#8217;s responsibilities for checking subforms or arguments not
mentioned in the paragraph.</p>
<p>
</p>
<a name="node_sec_6.4"></a>
<h2 class=section><a href="r6rs-Z-H-2.html#node_toc_node_sec_6.4">6.4&nbsp;&nbsp;Other kinds of entries</a></h2>
<p>If <i>category</i> is something other than &#8220;syntax&#8221; and
&#8220;procedure&#8221;, then the entry describes a non-procedural value, and
the <i>category</i> describes the type of that value.  The header line</p>
<p>
</p>

<p class=noindent></p>
<div align=left><tt>&amp;who</tt>&nbsp;&nbsp;&nbsp;&nbsp;condition type&nbsp;</div>
<br>
indicates that <tt>&amp;who</tt> is a condition type.  The header line<p>
</p>

<p class=noindent></p>
<div align=left><tt>unquote</tt>&nbsp;&nbsp;&nbsp;&nbsp;auxiliary syntax&nbsp;</div>
<br>
indicates that <tt>unquote</tt> is a syntax binding that may occur
only as part of specific surrounding expressions.  Any use as an
independent syntactic construct or identifier is a syntax violation.
As with &#8220;syntax&#8221; entries, some &#8220;auxiliary syntax&#8221; entries  carry a
suffix (<tt>expand</tt>), specifying that the syntactic keyword of the
construct is exported with level 1.
<a name="node_sec_6.5"></a>
<h2 class=section><a href="r6rs-Z-H-2.html#node_toc_node_sec_6.5">6.5&nbsp;&nbsp;Equivalent entries</a></h2>
<p>The description of an entry occasionally states that it is <i>the
same</i> as another entry.  This means that both entries are
equivalent.  Specifically, it means that if both entries have the same
name and are thus exported from different libraries, the entries from
both libraries can be imported under the same name without conflict.</p>
<p>
</p>
<a name="node_sec_6.6"></a>
<h2 class=section><a href="r6rs-Z-H-2.html#node_toc_node_sec_6.6">6.6&nbsp;&nbsp;Evaluation examples</a></h2>
<p>The symbol &#8220;&rArr;&#8221; used in program examples can be read
&#8220;evaluates to&#8221;.  For example,</p>
<p>
</p>

<tt>(*&nbsp;5&nbsp;8)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&rArr;&nbsp;&nbsp;40<p></tt></p>
<p>
means that the expression <tt>(* 5 8)</tt> evaluates to the object <tt>40</tt>.
Or, more precisely:  the expression given by the sequence of characters
&#8220;<tt>(* 5 8)</tt>&#8221; evaluates, in an environment that imports the relevant library, to an object
that may be represented externally by the sequence of characters &#8220;<tt>40</tt>&#8221;.  See section&nbsp;<a href="r6rs-Z-H-7.html#node_sec_4.3">4.3</a> for a discussion of external
representations of objects.</p>
<p>
The &#8220;&rArr;&#8221; symbol is also used when the evaluation of an
expression causes a violation.  For example,</p>
<p>
</p>

<tt>(integer-&gt;char&nbsp;<tt>#</tt>xD800)&nbsp;&nbsp;<tt>&nbsp;&amp;assertion</tt>&nbsp;<i>exception</i><p></tt>
means that the evaluation of the expression <tt>(integer-&gt;char
<tt>#</tt>xD800)</tt> must raise an exception with condition type
<tt>&amp;assertion</tt>.</p>
<p>
Moreover, the &#8220;&rArr;&#8221; symbol is also used to explicitly say that
the value of an expression in unspecified.  For example:
</p>

<tt>(eqv?&nbsp;&quot;&quot;&nbsp;&quot;&quot;)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&rArr;&nbsp;&nbsp;<i>unspecified</i><p></tt></p>
<p>
Mostly, examples merely illustrate the behavior specified in the
entry.  In some cases, however, they disambiguate otherwise ambiguous
specifications and are thus normative.  Note that, in some cases,
specifically in the case of inexact number objects, the return value is only
specified conditionally or approximately.  For example:
</p>

<tt>(atan&nbsp;-inf.0)&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;<br>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&rArr;&nbsp;-1.5707963267948965&nbsp;;&nbsp;approximately<p></tt></p>
<p>
</p>
<a name="node_sec_6.7"></a>
<h2 class=section><a href="r6rs-Z-H-2.html#node_toc_node_sec_6.7">6.7&nbsp;&nbsp;Naming conventions</a></h2>
<p>By convention, the names of procedures that store values into previously
allocated locations (see section&nbsp;<a href="r6rs-Z-H-8.html#node_sec_5.10">5.10</a>) usually end in
&#8220;<tt>!</tt>&#8221;.</p>
<p>
By convention, &#8220;<tt>-&gt;</tt>&#8221; appears within the names of procedures that
take an object of one type and return an analogous object of another type.
For example, <tt>list-&gt;vector</tt> takes a list and returns a vector whose
elements are the same as those of the list.</p>
<p>
By convention, the names of predicates&#8212;procedures that always return
a boolean value&#8212;end in &#8220;<tt>?</tt>&#8221; when the name contains any
letters; otherwise, the predicate&#8217;s name does not end with a question
mark.</p>
<p>
By convention, the components of compound names are separated by &#8220;<tt>-</tt>&#8221;
In particular, prefixes that are actual words or can be pronounced as
though they were actual words are followed by a hyphen, except when
the first character following the hyphen would be something other than
a letter, in which case the hyphen is omitted.  Short,
unpronounceable prefixes (&#8220;<tt>fx</tt>&#8221; and &#8220;<tt>fl</tt>&#8221;) are not
followed by a hyphen.</p>
<p>
By convention, the names of condition types start with
&#8220;<tt>&amp;</tt>&#8221;<a name="node_idx_262"></a>.</p>
<p>
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